Chip-scale devices refer to devices whose size is sufficiently small so that they can be easily integrated into an integrated circuit or chip, and are generally materialized in 25 mm or less. Examples of chip-scale devices include switches, sensors, gyroscopes, accelerometers, atomic clocks, etc. In order to manufacture these chip-scale devices, technology that can produce a required shape of a fine size without changing electrical, mechanical or chemical properties is required.
Thus, chip-scale devices are manufactured using MEMS (Micro Electro Mechanical System) based on the semiconductor process technology, and more specifically, chip-scale devices can be manufactured by precisely processing material such as silicon, glass and the like based on MEMS.
An atomic clock, which is one of the chip-scale devices listed in the above examples, is an electronic timing device produced on the basis of the natural resonance frequency of atoms. The atomic clock utilizes the phenomenon of emitting or absorbing electromagnetic waves corresponding to the difference in a changed energy level when the energy level of certain atom changes to another energy level, which provides a high level of accuracy.
The atomic clock may be constituted by a physical part for changing the energy levels of atoms and detecting them, and a control part for determining the time interval by using the detected energy levels.
Among them, the physical part may be constituted by including a light generating unit (for example, a laser diode such as VCSEL) for generating a laser beam having a specific wavelength, an optical element for focusing the laser beam generated in the light generating unit, a vapor cell where atoms are accommodated in a space isolated from the outside and the focused laser light is incident and emitted, and a photo-detecting unit (for example, a photodiode) which receives the light emitted from the vapor cell and detects a change in the energy level of the atom.
In order to operate the atomic clock to show an increased accuracy in the time interval, it is needed that a sealed space in which atoms can be contained and isolated inside the vapor cell is provided, and the temperature of the sealed space is generally kept constant so that atoms can have a relatively equal operating temperature.
On the other hand, in Non-Patent Document 1 (Microfabricated alkali atom vapor cells with in-situ heating for atomic-based sensors, Liew L, Proc. 3rd Int. Symp. Sensor Science, 2005), additional silicon structures beside a silicon body constituting a vapor cell is prepared, and the temperature of a cavity accommodating the atoms is controlled by the structures. However, in this case, there are such problems that it is difficult to keep the temperature inside the cavity to be constant in general, so that the accuracy of the atomic clock is deteriorated. Also, since it is required to form an additional silicon structure, the manufacturing process of the vapor cell becomes complicated.
Meanwhile, the vapor cell which is one of components of the physical part contains vaporized alkali atoms (for example, cesium Cs or rubidium Rb). If only alkali atoms are injected into the vapor cell and subject to optical pumping, a stable frequency detection becomes difficult due to the collision of the activated atoms to the vapor cell wall, the Doppler effect according to their high speed movements or the like. Thus, a buffer gas for appropriately restraining alkali atoms is injected together.
Reviewing prior techniques for injecting alkali atoms and a buffer gas into a vapor cell, in Non-patent Document 2 (Microfabricated alkali atom vapor cells, L.-A. Liew, APPLIED PHYSICS LETTERS, 2004,) alkali atoms are extracted from a sealed vessel based on the following reaction formula.BaN6+{alkali}Cl→BaCl+3N2+{alkali}
Specifically, according to Non-patent Document 2, barium azide BaN6 is mixed with cesium chloride CsCl or rubidium chloride RbCl and injected into a vapor cell filled with a specific buffer gas, and the reaction is then activated at a high temperature of 300° C. to extract alkali atoms of nitrogen N2, cesium or rubidium together with barium chloride BaCl. However, in the above method, there is a problem that coagulated barium chloride remains inside the vapor cell, and not only the buffer gas and the vaporized alkali atoms but also nitrogen are similarly generated.
In order to solve these problems, Non-patent Document 3 (Wafer-level filling of microfabricated atomic vapor cells based on thin-film deposition and photolysis of cesium axide, L.-A. Liew, APPLIED PHYSICS LETTERS, 2007) describes a method in which liquid cesium azide CsN3 is injected into a vapor cell filled with a specific buffer gas and sealed, and then liquid cesium azide is activated using ultraviolet light. According to Non-Patent Document 3, although solidified barium chloride does not remain inside the vapor cell, there is a problem that nitrogen is generated in addition to the buffer gas and vaporized alkali atoms.
On the other hand, Non-Patent Document 4 (Microfabrication of cesium vapor cells with buffer gas for MEMS atomic clocks, M. Hasegawa, Sensors and Actuators A, 2011) proposes a method of using a Cs dispenser. Specifically, according to Non-Patent Document 4, Cs pill (a mixture of CsCrO4 and Zr—Al alloy) is additionally accommodated and sealed in a vapor cell filled with a specific buffer gas, and then activated at a high temperature of 700° C., whereby only Cc atoms can be injected into the sealed vapor cell. However, according to Non-Patent Document 4, since an additional space for the dispenser is required in the vapor cell, the size of the vapor cell is increased, large power consumption occurs, and the productivity is reduced since Cs pill is injected and activated for each vapor cell.